This invention relates to adhesion promotion.
The invention is especially, though not exclusively, concerned with methods involving organosilane adhesion-promoters on glass, metal or other surfaces.
The use of organosilanes as adhesion promoters has been found to require especial care. In particular, the adhesion achieved is affected by the pH of the organosilane solution and the thickness of the film deposited on the substrate surface. An object of the present invention is to provide methods by which good adhesion can be achieved.
According to the present invention there is provided a method of promoting adhesion with a substrate surface wherein the pH of said surface is modified to initiate the action of an adhesion-promoting agent and thereby enhance adhesion at said surface.
The reaction required on a glass or metal surface to promote optimum adhesion has been found to be critically sensitive to the history of the surface in regard to transport, abrasion and/or cleaning. This history has been found to affect the pH of that surface and the efficacy of adhesion as promoted, for example, by organosilanes.
It is common for the suppliers of organofunctional silanes to specify that the conditions of use of the silanes should be such as to sustain two reactions, these being hydrolysis primarily, and condensation. The recommended pH range for the organosilane when used with glass is from 3.5 to 5, but observations have indicated that even when using solutions within this range, there are common and consistent fluctuations in adhesion within each batch of sample glass.
These observations led to a detailed analysis, and from this it was found that the variation in adhesion within the batch paralleled the variation in pH of pure water into which the sample glass was immersed prior to application of the organosilane. From this it was determined that the presence of detritus of glass from abrasion (during transport or otherwise) or machining, develops an alkali solution when the surface is wetted. Accordingly, it was concluded that the machining of glass or the abrasion of two sheets upon one another during shipping or otherwise, leaves a residue of detritus on the glass that affects the pH-value of the silane solution, and therefore the performance, of the adhesion promoter. It was observed, moreover, that even if the washing water used for the glass is copiously changed and the product flushed with pure water, an alkaline residue is generally left on the glass after drying. This residue, when wetted by the organosilane solution, reduces the pH value of the solution and renders the hydrolysis reaction, partially or fully, ineffective.
The residue in the case of glass is alkaline but for other surfaces it may be acidic, and in accordance with a feature of the present invention the pH of the residue may be monitored and a compensatory adjustment of the pH of the adhesion-promoter solution made to effect optimum adhesion. To this end, the pH of the washing water at the final rinse station may be monitored and acid or alkali added to the promoter solution to give the optimum performance. For example, where glass is involved, acetic or some other acid may be added to ensure that the active organosilane solution wetting the glass surface has a pH within the optimum range of 3.5 to 5 for the hydrolysis reaction to take place.
The matter of the pH value of the surface-residue as referred to above, is important for the effectiveness of powder coating of substrates. In the case of glass substrates, the alkalinity of the surface to which the coating powder is applied affects the hydrolysis reaction required for the silane to give good adhesion. But by first spraying the glass surface with an acid solution (for example, a 10% solution of acetic acid in pure water) and allowing it to dry, the alkalinity can be effectively counteracted. The dried acid is reactivated during the initial heating of the powder for the curing process, so as to balance out the alkalinity shown by the glass and thereby ensure that the powder-borne organosilane is fully active in providing good adhesion between the substrate surface and its resultant powder coating.
Clearly, where the substrate surface to be coated with the powder exhibits acidity, that surface can be sprayed with an alkaline solution for counteracting the acidity and ensuring good adhesion promotion.
Furthermore, in accordance with the invention the adhesion-promoting agent may be included with the coating powder so as to become active when the powder is heated for fusing. The powder may also include an acid or alkali to initiate the action of the adhesion-promoting agent at the substrate surface during the heating.